(a) Field of the invention:
The present invention relates to a zoom finder.
(b) Description of the prior art:
Known zoom finders are arranged that the apparent size of the measuring field frame thereof does not vary at the time of zooming. Therefore, when a known zoom finder is used with an active-type automatic focusing camera, the measuring field indicated on an object by the measuring field frame becomes large in the wide position and small in the teleposition though the size of the measuring beam spot on the object does not vary.
In an active-type automatic focusing camera, it is conventional to utilize a projected beam of light to illuminate an object to be photographed. A detector lens is employed to measure the lens upon detection of the light reflected to provide data for automatically focusing the camera lens. If the light from the projector does not correctly fall on the object, the distance will not be accurately measured so that the auto focus device will not operate to provide a clear picture. In a camera provided with a zoom finder where the finder operates between a wide angle and a telephoto position, the focusing beam light from the project lens, called the "measuring beam spot", can easily be improperly located when a shift is made between the side angle and telephoto positions of the zoom finder.
Thus, in case that the size of the measuring field frame is adjusted to the size of the measuring beam spot in the state of the wide position, when photographing two persons who stand side by side for example, it apparently looks as if the measuring beam passes through the space between the two persons, when zoomed to the teleposition, in spite of the fact that the measuring beam is virtually striking upon the two persons sufficiently. Hence, to focus on the object accurately, it is unavoidable to put either of the two persons into the measuring field frame and, consequently, a photograph is often taken in such state that one of the two persons comes to a position near the center of the field in spite of the fact that it is actually possible to take a photograph by putting the two persons to the central portion of the field in a well balanced state.
On the other hand, in case that the size of the measuring field frame is adjusted to the size of the measuring beam spot in the state of the teleposition, it apparently looks, when zoomed to the wide position, as if the measuring beam is sufficiently striking upon the two persons in spite of the fact that the measuring beam is virtually passing through the space between the two persons and, as a result, the photograph often becomes out of focus.
It will be apparent, therefore, that in a circumstance where a user is photographing two objects, for example, two persons standing apart from each other but side by side, the measuring field frame, which is the focusing frame, is so adjusted as to be coincident with the diameter of the measuring beam spot at a wide angle position so that the focusing frame will overlap or cover both of the objects. In this condition, the light beam emitted from the camera will fall on both of the objects and it is therefore possible for the camera to correctly measure the distance to the objects. However, when these objects are to be photographed at the tele position by zooming the lens system from the wide angle position to the telephoto position, the finder will also be set at the telephoto position and the images of the objects observed through the finder will be enlarged and the space between the objects will increase in width. However, in the conventional camera of this type, the image size of the measuring field frame remains unchanged since the actual distance from the camera to the objects does not vary. Accordingly, the light beam for measuring the distance will still fall on both of the objects in the same manner as when the focusing finder is adjusted to the wide angle position. However, many photographers, particularly those of limited experience for whom these cameras are specifically designed, have tended to misjudge the location of the light beam distance measuring spot as it appears that it does not fall on both of the objects when the finder is zoomed to the telephoto position. In this circumstance, many users will shift the camera so that the light beam spot will fall on one of the objects which procedure can result in an unbalanced and therefore less than satisfactory photograph. On the other hand, when the focusing frame is adjusted in the telephoto position and then switched to the wide angle position through the zooming operation, the image of the measuring field frame, the focusing frame, is actually located between the two objects in the view of the user while in fact the beam is actually illuminating both objects so that the camera will measure the distance correctly and properly focus the camera. Also in this case, when the user switches from the telephoto position to the wide angle position, again, the actual distance between the camera and the objects is unchanged so that a correct measured distance will be effected. Unfortunately, when the measuring field frame is adjusted with the light beam spot in the telephoto position and then switched from the telephoto position to the wide position by zooming the lens system, the image of the measuring field frame, the focusing frame, is actually located between the object but since the focusing frame is located so as to cover both of the objects the camera will be capable of correctly measuring the distance but, as noted above, the user is likely to misjudge the operation of the camera.
It is usually the condition where the zooming is effected from the telephoto to the wide angles that errors effected by the user occur since, in this circumstance, the measuring light beam only apparently is in the correct position for measuring distance when in fact, in the circumstance where two people are standing side by side the measuring light beam will fall between.
A zoom finder disclosed in Japanese published examined patent application No. 9389/58 is known as a zoom finder whose lens system has a distribution of refractive power resembling the distribution of refractive power in the preferred embodiment of the zoom finder according to the present invention, in other words, a zoom finder whose lens system forms a Galilean telescope comprising three lens units, i.e., a positive, negative and positive lens units. However, in case of said known zoom finder, any consideration is not given to indication of a measuring field frame and/or measuring beam spot into the field and, therefore, any solution is not given to the afore-mentioned disadvantage of known zoom finders.